This invention relates to a liquid crystal display device that is adapted for illumination by ambient light. More particularly, this invention relates to such liquid crystal display device that includes a reflective holographic optical element that redirects diffuse light through a liquid crystal panel in a preferential reflection pattern. In one aspect of this invention, the reflective holographic optical element is a transflector for alternately backlighting the liquid crystal display.
A typical liquid display device comprises a liquid crystal panel for forming a display that is viewed through a front side of the panel. The liquid crystal panel comprises a layer of liquid crystal material sandwiched between front and back transparent polymer or glass plates and between front and back polarizers having perpendicular axes of polarization. Transparent electrodes are affixed to the inner surfaces of the transparent plates to alter the light transmission properties of the liquid crystal material in the adjacent region of the panel. In this manner, the electrodes define pixels that create the display. As used herein, pixels refer to a region of the liquid crystal panel that forms a light or dark area of the display. A common display is formed of pixels that are dots arranged in a regular array. Another common type of display comprises pixels arranged in a "FIG. 8" and selectively switched to form alphanumerical characters.
In the absence of an applied electric field, polarized light is admitted through one polarizer and reoriented by the liquid crystal layer to pass through the opposite polarizer, thereby causing the panel to be transparent and creating a bright pixel for the display. However, an electrical potential applied between the electrodes alters the intermediate liquid crystal material to prevent light from passing through the panel, thereby creating an opaque region in the panel that appears as a dark pixel. Thus, by selectively applying electrical current to the electrodes, the pixels are individually switched between a transparent or bright state and an opaque or dark state.
While the liquid crystal panel creates a display by locally altering the light transmission properties of the pixels, the liquid crystal panel does not generate the light needed for viewing the display. There are two basic approaches for providing light for the display. One approach utilizes ambient light and comprises a reflector facing the back side of the liquid crystal panel. At a transparent region, ambient light is filtered by the front polarizer, traverses the panel, is reflected by the reflector, and retraverses the panel to create a bright pixel. Of course, at a region that is opaque, light does not traverse the panel, creating a dark pixel. In a second approach, the device comprises a light source for illuminating the back side of the liquid crystal panel, whereupon the light is screened by the back polarizer and traverses transparent regions of the liquid crystal panel to create bright pixels. Attempts have been made to combine these two approaches in a device by employing a transflector between the back side of the liquid crystal panel and a light source. The transflector reflects ambient light for operation under ambient light conditions and transmits light from the light source for operation in that mode. One problem with conventional transflectors is that the efficiency of reflection, that is, the ratio of reflected light to incident light, is necessarily reduced by the transmission properties of the transflector, and, similarly, the transmission efficiency is reduced by the reflection properties of the transflector, so that, for example, a transflector that is 50 percent 50 percent transmissive reflects only about 50 percent of the light, thereby reducing the proportion of viewable light in both modes.
Whereas in backlighting, a desired brightness is achieved by providing a light source of a predetermined intensity, ambient lighting is dependent upon the brightness of the surrounding light, which may originate from any directions and varies in intensity. To provide adequate brightness under such varying conditions, reflectors have been developed that produce a diffuse reflection. In contrast to a mirror that produces a highly directional reflection based upon the angle of incidence of the light, a diffuse reflector reflects light uniformly over a wide range of angles, so that the intensity of reflected light is relatively independent of the direction of the ambient light. However, such diffuse reflection necessarily reduces the intensity of light perceived by the viewer. Moreover, the apparent brightness of the display is further diminished by losses in double transmission through the panel, for example, by the polarizers. As a result, ambient illumination suffers from diminished apparent brightness perceived by the viewer. This is particularly noticeable in situations of low ambient light. Therefore, there is a need to increase the efficiency of reflected light that is available for viewing the display to enhance the apparent brightness as perceived by the viewer.